Static and dynamic models of spatial orientation

Localization of the subjective vertical during body tilt in pitch and in roll has been extensively studied because of the relevance of these axes for aviation and control of posture. We performed the first measurement of static orientation in -90º to +90º pitch, roll and recumbent yaw using the same subjective orientation task in all axes.

Subjects’ tasks were to point a gravity-neutral probe to the gravitational vertical (haptically indicated vertical) and to report verbally their perceived tilt. Subjects underestimated their body tilts in recumbent yaw and pitch and overestimated their tilts in roll. The haptic settings for pitch and roll were consistent with data in the literature. Our data constitute the first tri-dimensional assessment of the subjective vertical using a common measurement procedure and provide the basis for the tri-axial modeling of vestibular function.

We have used this data set to develop a tri-axial model of spatial orientation. The model attempts to capture the mechanics of otolith organ transduction of static linear forces and the perceptual computations performed on these sensor signals to yield subjective orientation of the vertical direction relative to the head.

Our model differs from other treatments that involve computing the gravitoinertial force (GIF) vector in three independent dimensions. The perceptual component of our model embodies the idea that the central nervous system processes utricular and saccular stimuli as if they were produced by a GIF vector equal to 1 g even when it differs in magnitude, because in the course of evolution living creatures have always experienced gravity as a constant. We determine just two independent angles of head orientation relative to the vertical which are GIF-dependent, the third angle being derived from the first two and being GIF-independent.

Somatosensory stimulation is used to resolve our vestibular model’s ambiguity of the up-down directions. Our otolith mechanical model takes into account recently established non-linear behavior of the force-displacement relationship of the otoconia, and possible otoconial deflections that are not co-linear with the direction of the input force (cross-talk). The free parameters of our model relate entirely to the mechanical otolith model. They were determined by fitting the integrated mechanical/perceptual model to subjective indications of the vertical obtained during pitch and roll body tilts in 1 g and 2 g force backgrounds and during recumbent yaw tilts in 1 g.

The complete data set was fit with very little residual error. A novel prediction of the model is that background force magnitude, either lower or higher than 1 g, will not affect subjective vertical judgments during recumbent yaw tilt. These predictions have been confirmed in recent parabolic flight experiments.

In 1 and 1.8 g test conditions, subjects were able to indicate both the subjective vertical and the amplitude of the body rotation reasonably accurately. By contrast in 0 g, when indicating the subjective vertical, they aligned the pointer with the body midline and kept it nearly aligned with their midline during the subsequent body tilts. They also reported feeling supine throughout the 0 g test periods.